Pincer-type complexes structure

Some other intermolecular C-H activations involving the NHC ligand have been observed during the synthesis of particular NHC-containing pincer -type complexes also called CCC-NHC complexes. In addition to zirconium- and rhodium-based complexes (210) and (211)/ several examples involving palladium of general structure (271) have been synthesized. Whereas Faller... 

Espinet et air reported a rather nice series of 2,6-dipicolinate derivatives 84 whose structure is reminiscent of pincer-type complexes. The first thing to note is that none of the Pd complexes of dipicolinate (84a X = 0)... 

Figure 80 Chemical structures of the pincer-type complexes (191).

Complexes of type (90) represent a new class of zirconium hydrocarbyl derivatives in which a strong metal carbene interaction in a pincer hgand structure is formed. The crystal structure of such complexes shows a short metal carbon link common to two fused four-membered rings which are almost coplanar. 

Pincer complexes with other chelating donors other than phosphorus are also useful as SRPCs. NCN- or SCS-structures are likely to be less stable than PCP-pincer complexes. Therefore, catalytic turnover nsing these precatalysts is possible at lower reaction temperatures. Generally, precatalysts 194-198 were tested in type 1 reactions with activated substrates (Table 2.10, Figure 2.25) [250-256]. Attachment of pincer complexes to various polymeric supports enabled the design of recyclable systems for type 1 reactions [250]. Moreover, lower stability of NCN- or SCS-pincer complexes ensures higher effective concentrations of catalytically active species precatalyst 195 was shown to serve well in reactions with activated aryl chlorides (Table 2.10, entry 3), typical substrates for type 2 processes [252],... 

Unexpected Formation of Chiral Pincer CNN Nickel Complexes with P-Diketiminato-Type Ligands via C-H Activation. Synthesis, Properties, Structures, and Computational Studies... 

Steinke et al. synthesized a new type of PCP pincer ligand where the central carbon donor is a saturated NHC (Figure 4.11) [45]. These structures showed a considerable twisting of the ligand framework above and below with respect to the square plane of the metal complex and the PCP donors. In solution, this twisting is not fixed, as the two possible chiral conformations exchange first on the NMR time scale. 

A partioilar case of this type of metal-ligand cooperation (MLC), studied by Milstein et al, is based on dearomatization/aromatization processes on the ligand. These studies have been comprehensively covered in several recent reviews, but due to their relevance and fundamental importance they have been, although briefly, also included herein. These authors found that pyridine-based PNP and PNN (structures 33 and 34, Fig. 20), and bipyridine-based (structure 35, Fig. 20) pincer ligands can stabilize coordinatively unsaturated complexes. 

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